1. Field of the Invention
The present invention relates to miniature bearing failure sensors and, more particularly, to micromachined mechanical resonators for sensing vibrations caused by bearing failures.
2. Description of the Prior Art
One of the most critical problems confronting users of rotating machinery has been the early detection of bearing failures well before serious damage to the bearings occur. Those concerned with these problems have employed a variety of devices designed to detect the onset of bearing failures or actual failures by sensing vibrations at peak frequencies corresponding to bearing rotation. More specifically, it has been the general practice to mount on a machine to be monitored one or more mechanical resonators corresponding to a frequency equal to the frequency of passage of ball bearings or roller bearings or gear teeth or like rotating devices. When damage occurs, the bearing or gear teeth create vibrations which are sensed by the mechanical resonators, permitting corrective action to be taken before major mechanical problems develop.
Examples of prior art systems for sensing bearing failures may be found in the following U.S. Pat. Nos.: 4,237,454; 4,089,055; 4,729,239; 4,763,523; 4,528,852; 4,078,434; 4,493,042; 4,479,389; 4,768,380. Although these and other prior art devices have served the purpose, they have not proved entirely satisfactory under all conditions of service for the reason that considerable difficulty has been experienced in developing reliable, low-cost devices that have sufficient sensitivity to detect very small vibrations that occur in the early stages of failure. Also, the prior art devices lack the ability to detect bearing failures in small-scale machines. Still further, even the smallest prior art sensors to date are usually too massive to be used on bearings located in inaccessable areas.
The '239 patent discloses a highly sensitive ball bearing tester for testing miniature bearings. This device includes a test structure that is significantly larger than the bearing being tested. It requires that the miniature bearing under test be isolated from its associated structure and be mounted on the test structure. As such, this ball bearing test structure is not intended for use in monitoring machinery during actual operation.
A typical system which monitors bearings and other rotating equipment while in actual use is disclosed in the '454 patent. The disclosed system which monitors rotating equipment from a central station includes self-contained and self-powered monitors located on each piece of equipment. Each monitor has a fault detector and a transmitter with the ability to signal a local fault conditioned to a central control point. Electrical power in each unit is derived from a tuned mechanical resonator operating at the rotational frequency of the equipment which is driven by the fundamental unbalanced component of machine vibration. The defect detection and transmitter circuitry may be integrated circuits so that the resonator can generate sufficient power to operate the circuitry. The transmitter and the monitor are triggered by a defect signal produced by a vibration signal processing circuitry and transmits to the central station a short duration radio signal that is modulated to uniquely identify the equipment with the default condition. At the central station there is a receiver for all monitor units which determines the location or identity of the malfunctioning equipment. Vibrations are sensed by the mechanical resonator which includes a cantilevered beam having a piezo-crystal attached thereto. Consequently, this monitor includes a relatively large-scale resonator device capable of producing sufficient electrical power to drive the detection and transmission circuitry. Relatively large-scale monitors of this type suffer in that they are not readily adaptable for use in monitoring miniature bearings due to lack of sensitivity. Additionally, the small changes in vibration intensity that occur in the early stages of failure will also go undetected by large-scale monitors also due to the lack of sensitivity. Still further, large, massive resonators cannot be used in many small inaccessable locations. Although there has been a long recognized need for more sensitive bearing failure sensors capable of providing sufficient warning at the onset of bearing failure, no practical prior art system for doing so has yet been devised.